Modeling and Design of Thin-Film Conducting Polymer Gas Sensors Based on Resonant RF Circuit Structures

Conducting polymer thin-films have often been used as the gas sensitive layer in chemiresistors as they have several favorable properties over other gas sensitive materials such as metal oxides. The ability to operate at room temperature and the ability to dope polymers to specific conductivities has made conducting polymers attractive as a sensing element. However, both suffer from issues dealing with low sensitivity and low selectivity to specific gases. Typically, sensors of this type measure the change in resistance of the conducting polymer layer that occurs when it interacts with a gas. Recently, the capacitive nature of conducting polymer sensors has been explored and several studies have shown that measuring the capacitive behavior of conducting polymer sensors can lead to more sensitive sensors as well as the possibility of increasing selectivity due to the ability to measure dielectric behavior which may differ among different gases. In practice, these gas sensitive layers are often deposited onto interdigital electrodes which consist of a series of coplanar fingers which are alternately connected. In this work, the sensitivity of thin-film conducting polymer sensors based on interdigital capacitors is explored using the partial capacitance method. This method allows for the examination of the capacitance contribution from very thin dielectric layers above the surface of an interdigital capacitor. One of the limitations of the partial capacitance method is that the calculation of the capacitance of a very thin layer involves the division of two very large almost equal numbers which increase as the thickness of the dielectric layer decreases. This can lead to increased computational time and can limit the study of very thin layers based on the number of significant digits that must be used in the calculation. In this work, an approximate capacitance model is proposed based on an equivalent parallel plate capacitance which closely agrees with the conformal mapping method for very thin dielectric layers. This allows for the study of dielectric layers of virtually any thickness using standard double floating-point precision arithmetic. Another limitation of the partial capacitance method is that it can only be used for analyzing IDCs with multiple dielectric layers when the values of the dielectric constants are either monotonically increasing or decreasing with respect to the electrode plane. In this paper, a mixed partial capacitance method is proposed which for the first time allows for the analysis of mixed dielectrics with arbitrary dielectric constants.;The partial capacitance method is then used to examine the sensitivity of conducting polymer gas sensors based on key geometric properties of the interdigital capacitor and the gas sensitive conducting polymer layer. Several test sensors were constructed using a commercially available polypyrrole solution and were found to be in close agreement with the partial capacitance method.;Finally, the partial capacitance method is used to design gas sensors based on RF resonant structures such as microstrip ring and linear resonators. Key geometric properties of the design of such sensors are identified in order to obtain the maximum sensor response using equivalent circuit models and finite element analysis.